1. Field of the Invention
The present invention relates to a switching device that switches signals, especially signals that are assembled to form packets, and a network employing such a switching device; and to a simplified switching control method, and to a communication method employing such a switching method.
2. Related Background Art
Various types of conventional switching devices and methods have been studied and discussed. The structure of a packet that is switched by one of these packet switching devices is shown in FIG. 1. In FIG. 1, an address portion 109 holds output terminal instructions that specify the output terminal for the packet, while held in data portion 110 are the data that are carried in the packet.
FIG. 2 is a diagram of a first conventional example, a crossbar switching device that has N input terminals and N output terminals. In FIG. 2 decoders 111, each of which reads the address portion of a packet, transmit to a control unit 115 output terminal signals instructing the terminal equipments at which packets are to be output. Each FIFO (First In First Out) queue (hereinafter referred to as an "FIFO") 112 temporarily stores received packets, and, under the control of the control unit 115, outputs the packets across its output line 116 in the order in which they were received. Input lines 113 are employed to carry to the input terminals of switches 114 packet signals that are output at the FIFO 112. The switches 114 are used to select whether or not packet signals that are received along the input lines 113 are to be output to the output lines 116. In consonance with the output from the decoders 111 the control unit 115 receives, it determines when to read data from the FIFO 112 and when to open and when to close the switches 114. The output lines 116 are employed, along which packet signals that are output from the switches 114 are supplied to the output terminals.
In the crossbar switching device, the control unit 115 opens or closes the switches that are connected to required output terminals, and thus provides routing control for the changing of the output terminals to which signals are forwarded. Also, when a routing disagreement arises, as when there are requests for the output at the same output terminal of input data that have been received at multiple input terminals, i.e., when a so-called output conflict occurs, the control unit 115 performs arbitration control to determine, from among the input at the plurality of input terminals, the data that are to be output. The performance of these control processes constitutes the switching operation.
However, in the first conventional example, which has N input terminals and N output terminals, N.times.N switches are required. As a result, the size of the hardware is greatly increased.
In addition, in the first prior art, N output terminals of the switches, by which a plurality of input lines and output lines are connected together, are connected to a single output line. As a result, length of connecting lines is extended and this causes the wiring delays and an increase in the floating capacity of wiring. As the number N of the input terminals becomes greater, it is difficult to increase the operating speed of the switches. Thus, the first prior art can not adequately handle the high speed switching that is required for input packet signals.
Further, in the first prior art, the occurrence of output conflicts must be detected for all inputs at all the input terminals, and arbitration control must be performed. Therefore, because of the performance of this control, the size of the hardware for the control unit is increased.
FIG. 3 is a second conventional example that has been proposed to overcome the shortcomings of the first conventional example. For the design of a switching device, a plurality of 2.times.2 switches, each of which has two input and two output terminals, are connected together at multiple steps. In FIG. 3, 2.times.2 switches 117 through 128, each of which has two input terminals and two output terminals, perform two functions: a "straight" function for directly connecting input terminals to output terminals, and a "crossing" function in which switching paths intersect before they connect input terminals to output terminals. These twelve 2.times.2 switches are connected to form a shuffle network and thus provide an omega switching device that has eight input terminals and eight output terminals.
FIG. 4 is a diagram showing the internal structure of one of the 2.times.2 switches 117 through 128 that have two input terminals and two output terminals. In FIG. 4, a decoder I 129 and a decoder II 130 read the address portion of a received packet and notify a control unit 135 through which output terminal the packet is to be sent. An FIFO I 131 and an FIFO II 132 temporarily store received packets, and under the control of the control unit 135, they output the packets to respective selectors I and II, 133 and 134, in the same order in which the packets were input. The selectors I and II, 133 and 134, select the FIFO I 131 or II 132 in which is stored a packet signal that is to be output to an external destination. When the above mentioned straight function is performed, the selector I 133 selects the FIFO I 131, and the selector II 134 selects the FIFO II 132. When the crossing function is performed, the selector I 133 selects the FIFO II 132 and the selector II 134 selects the FIFO I 131.
In the second prior art, the number of required 2.times.2 switches is N log N-N/2 (2 is the log). Although this number is smaller than the N.times.N in the first prior art, each of the 2.times.2 switches requires decoders, FIFOs, a control unit and selectors, and thus the size of its hardware is large. In addition, in the second prior art, even if different input terminals are not connected to the same output terminal, an input terminal may not be connected to a desired output destination when a specific connection condition exists at of the other input terminals, i.e., a so-called blocking phenomenon will occur. For example, when input terminal 5 in FIG. 3 is connected to an output terminal 3, the 2.times.2 switch 117 is placed in the crossing state. However, in order to connect input terminal 1 to output terminal 1, the 2.times.2 switch 117 must be placed in the straight state. As a result, the blocking phenomenon occurs.
In a switching device, shown in the first and the second prior art, that employs electric switches, electric devices that can be switched at high speed are required for high speed processing. These electric devices are very expensive, and as a result the cost of manufacturing a switching device is increased. As one way to provide a high speed packet signal switching device, a switching device has been discussed that converts a packet signal into an optical signal and then switches the converted signal.
A third conventional example is of this type. In it, optical fibers are used to connect together, at multiple steps, a plurality of 2.times.2 optical waveguide switches, which have the same function as those in the second prior art, to provide an 8.times.8 switching device. In FIG. 5 are a schematic diagram and a cross sectional view of a total reflection InP optical switch, which is employed in the third prior art and which is one of crossing optical switches that are employed as 2.times.2 optical waveguide switches. In the total reflection InP optical switch, a carrier is introduced in a crossing portion where two optical waveguides cross each other to vary the refractive index in a refractive index variable region. Thus, an optical signal that enters the crossing portion is either transmitted or is totally reflected, and the switching is thereby performed. The variance of the refractive index that is induced by introducing a carrier is derived from band filling effects, during the shifting of the bands, according to which a change in a refractive index is increased as the wavelength of an incident light approaches the wavelength of an absorption end.
The introduction of a current into a refractive index variable region is performed with a carrier closing effect by using a p-InP clad layer and an n-InP substrate that have a large band gap, and by the constriction of a current in a Zn diffusion region. An InGaAsP cap layer is provided to obtain a preferable ohmic contact with an electrode. An optical switch must reduce the transfer loss for an optical signal and increase an optical quenching ratio (reduce cross talk). The refractive index must vary greatly in order to increase the optical quenching ratio. However, the above described optical switch is provided by using the band filling effect, and as the wavelength of incident light nears the wavelength at the absorption end, the transfer loss and the variance in the refractive index are increased. Setting the wavelength of incident light is difficult because, depending on the setting for the wavelength, a choice must be made as to whether a change in the refractive index should be reduced to prevent transfer loss and an increase in cross talk should be ignored; or whether a change in the refractive index should be increased to reduce cross talk and an increase in the transfer loss should be disregarded. Especially when 2.times.2 switches are to be connected, multiple steps for connection can not be employed due to a problem that arises from a trade-off involving transfer loss and a cross talk, and a large switching device can not be provided. In addition, since the response speed for the changing of the switches is limited by the lifetime of an injected carrier, high speed switching can not be performed.
In consideration with the above problems, the present inventor proposed the following switching device and switching method.
FIG. 6 is a diagram illustrating the structure of that switching device. The switching device has eight input terminals and eight output terminals and is constituted by eight variable wavelength transmission units I through VIII, which employ a tunable laser diode (TLD), and eight reception units I through VIII, which employ a photodiode (PD). In FIG. 6, each of the decoders I 136 through VIII 143 reads the address portion of an input packet, and transmits a destination instruction to a control unit 177 for the output terminal at which the packet is to be output. First In First Out queues (hereafter referred to as "FIFOs") I through VIII 144 through 151 temporarily store the packets they receive and then, under the control of the control unit 177, transfer the packets to the respective variable wavelength transmission units in the order in which they were received. Variable wavelength transmission units I 152 through VIII 159 are controlled by a wavelength control unit 179 in the control unit 177. They convert the packet signals that they receive from the FIFO I 144 through FIFO VIII 151 into optical signals having specific wavelengths and emit these signals to a star coupler 160. The star coupler 160 merges all the light of the wavelengths that are emitted from these eight variable wavelength transmission units 152 through 159 and then emits the light to eight filters 161 through 168. The filters I 161 through VIII 168, each functions to transmit only an optical signal having a fixed wavelength and to filter out optical signals having the other wavelengths. The transmitted wavelength .lambda.1 is set for the filter I 161; .lambda.2, for the filter II 162; .lambda.3, for the filter III 163; .lambda.4, for the filter IV 164; .lambda.5, for the filter V 165; .lambda.6, for the filter VI 166; .lambda.7, for the filter VII 167; and .lambda.8, for the filter VIII 168. The reception units I 169 through VIII 176 employ a photodiode to convert into electrical signals the optical signals that have the given wavelengths that are transmitted through the respective filters I 161 through VIII 168, and output each of the electric signals to a specific output terminal I to VIII. The control unit 177, which initiates the switching operation by the switching device, is constituted by an arbitration control unit 178 and the wavelength control unit 179. In consonance with instructions that are output by the decoders 136 through 143, the arbitration control unit 178 resolves output conflicts, between packets that have been input at the input terminals I through VIII, for each of the output terminals I through VIII at which the packets are to be output. The arbitration control unit 178 then issues an instruction to the wavelength control unit 179 that is consonant with the arbitration result. Upon the receipt of the instruction from the arbitration control unit 178, the wavelength control unit 178 controls the transmission wavelengths of the variable wavelength transmission units 152 through 159.
With the above described arrangement, since the eight filters I 161 through VIII 168 are so set that the wavelengths of optical signals that are transmitted through them are different, the wavelengths of the optical signals that enter the individual reception units differ and are independent. Therefore, a routing function for changing the output terminal for signal output can be provided by varying the transmission wavelength of the variable wavelength transmission units 152 through 159.
However, with this arrangement, arbitration control is required for packets that are input at all the input terminals.
Further, the transmission wavelength must be adjusted to a given wavelength for each packet in consonance with an instruction from the arbitration control unit. Therefore, when, for example, a packet has been forwarded with the shortest wavelength, and when a succeeding packet that is to be output has the longest wavelength, the magnitude of the change in the transmission wavelength that is performed by the variable wavelength transmission unit is great.
In this case, high speed wavelength control is required, so that the size of the hardware is increased or the time required for the processing to change the wavelength is extended.
As a similarly structured switching device, a technique called BHYPASS is disclosed in "Matthew S. Goodman; October 1989, IEEE Communication Magazine, pp. 27-35". While its arrangement is similar to that in FIG. 6, the device employs the Batcher-Banyan algorithm to control in advance the routing of packets, so that packets which are to be sent to the same output terminal are not input to a plurality of variable wavelength transmission units at the same time. The device also controls the wavelengths of the variable wavelength transmission units. With this arrangement, the advance control is required to ensure that packets which are to be sent to the same output terminal are not input simultaneously to a plurality of wavelength transmission units. In addition, the wavelengths of the wavelength transmission units must be adjusted in consonance with the packet destinations.